The petroleum industry uses 6.9 quadrillion BTU's of energy per year, 40% of this energy is used for distillation nation wide. The energy consumption used in the distillation process is even larger worldwide. Ethylene and propylene (olefins) are two of the largest commodity chemicals in the U.S. and are major building blocks for the petrochemicals industry. These olefins are mostly separated by cryogenic distillation that demands extremely low temperatures and high pressures. Over 75 billion pounds of ethylene and propylene are distilled annually in the U.S. at an estimated energy requirement of 400 trillion BTU's.
The largest potential area for energy reduction is in the cryogenic isolation of the product hydrocarbons from the reaction by-products, methane and hydrogen. This separation requires temperatures as low as −150° F. and pressures exceeding 450 psig.
Light hydrocarbon olefin/paraffin separations are dominated by cryogenic distillation technology at an estimated 1.2×1014 BTU's expended annually. In addition, there is enormous capital and operating costs associated with distillation. This has motivated an appreciable amount of effort towards pursuing alternative olefin/paraffin separation technologies. In the past decade, reactive or selective membranes for the olefin and paraffin separation had been widely investigated. However, the difficulty of long term stability associated with these facilitated transport membranes is a major obstacle.
Recently, the possibility of capillary condensation using a porous structure to separate light gases was explored, and the potential of using non-selective membrane for the olefin/paraffin separation was shown (see U.S. Pat. No. 6,039,792). In 2003, work was reported on using non-selective and non-porous membrane as structured packing to replace the distillation column for water-isopropanol separation and also suggested the possibility of using this technology for light hydrocarbon mixture separations (reference: Zhang, G. et al., “Hollow fibers as structured distillation packing,” Journal of Membrane Science 2003, 215, 185-193).
Non-selective micro-porous membranes have been used as a barrier material in membrane contactors for vapor/liquid or liquid/liquid mass transfer, desalination, concentrating fruit juice and enriching the oxygen in blood during open-heart surgery (reference: “Hollow fiber membrane contactors”, Journal of Membrane Science, 159 (1999) 61-106).
A unique feature of micro-porous membranes is a large surface area within a small volume, normally more than 3000 m2/m3, that provides an increased rate of mass transfer at least 10-20 times faster than the conventional tray and structured packing materials (creating an intimate contact between liquid and vapor phases). Although the high efficient Sulzer Chemtech® structured packing materials (AG Corporation), such as 250 Y/X, have conquered the chemical industry globally in the past twenty years, it has not been used in the C3 and C4 splitters since they require high liquid loads where structured packings typically deteriorate in performance. An advantage of using hollow fibers as structured packing is that the liquid and vapor velocity are not interrupted by the two-phase fluid mechanics. Therefore, the column packed with hollow fibers can operate above the normal flooding limits and below the normal loading limits.
However, the general requirement for use of a porous membrane in membrane distillation is that the membrane should not be wetted by the process liquids, because it is believed that the membrane wall once filled with liquid will increase the mass transfer resistance between the liquid and vapor phase, as well as reduce the flux of process fluid passed through the membrane wall. Therefore, historically it has been a requirement that the membrane used in the membrane distillation exhibit non-wetting characteristics (reference: “Membrane distillation”, K. W. Lawson et al., Journal of Membrane Science, 124 (1997) 1-25; “Hollow fiber membrane contactors”, Journal of Membrane Science, 159 (1999) 61-106; and, “Designing hollow fiber contactors”, M. C. Yang et al., AlChE Journal, 32 (1986) 1910-1916).
The present invention includes the uses of a non-selective mesoporous and/or microporous membrane to separate olefinic mixtures from light vapor byproducts at higher temperatures and lower pressures than are currently required.
Various objectives, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objectives and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.